Method for depositing a chromium-comprising passivation layer on a zinc-comprising coating

ABSTRACT

The present invention refers to a method for depositing a chromium-comprising passivation layer on a zinc-comprising coating, wherein the zinc-comprising coating additionally comprises Fe, Sn, Mn, or mixtures thereof. The method utilizes a passivation composition comprising 0.001 mg/L to 200 mg/L, based on the total volume of the passivation composition, of at least one corrosion-inhibiting agent selected from the group consisting of unsubstituted azole compounds, substituted azole compounds, unsubstituted aliphatic organic acids with at least one mercapto-group, substituted aliphatic organic acids with at least one mercapto-group, salts, and mixtures thereof.

FIELD OF THE INVENTION

The present invention refers to a method for depositing achromium-comprising passivation layer on a zinc-comprising coating,wherein the zinc-comprising coating additionally comprises Fe, Sn, Mn,or mixtures thereof. The method utilizes a passivation compositioncomprising 0.001 mg/L to 200 mg/L, based on the total volume of thepassivation composition, of at least one corrosion-inhibiting agentselected from the group consisting of unsubstituted azole compounds,substituted azole compounds, unsubstituted aliphatic organic acids withat least one mercapto-group, substituted aliphatic organic acids with atleast one mercapto-group, salts, and mixtures thereof.

BACKGROUND OF THE INVENTION

Different methods are known in the prior art to protect metal substratesfrom corrosive environmental influences. For example, a protectivecoating of a metal or metal alloy is applied on the metal substrate,which is a widely used and well-established method.

Among such protective coatings, the deposition of a zinc coating orzinc-comprising coating additionally comprising nickel (i.e. a zinc andzinc-nickel galvanization layer, respectively) on metal substrates (inparticular iron substrates) appears to be the most prominent approach.

In zinc-comprising coatings additionally comprising nickel, the chemicalelement nickel is mandatory. However, nickel and nickel ions as utilizedin respective galvanization compositions are hazardous to theenvironment and health. Thus, there is an ongoing demand to providealternative protective coatings being free from nickel. A promisingalternative protective coating is a zinc-comprising coating furthercomprising for example iron.

Typically, such coatings are additionally protected (post-treated) by socalled conversion coatings (often also referred to as passivationlayers). Such conversion coatings typically comprise insoluble compoundsas a result from reacting the protective coating with a conversiontreatment solution (i.e. passivation composition).

In many cases the passivation composition comprises trivalent chromiumions in an acidic solution. During the passivation process, theprotective coating is slightly dissolved such that metal ions arereleased, for example zinc ions. In turn, these metal ions react withcompounds in the passivation composition. For example, if a zinc coatingor zinc-comprising coating with nickel is contacted with such acomposition, some of the zinc and/or nickel will dissolve and formrespective ions thereof. Without applying any current, a chromium (III)hydroxide passivation layer or a p-oxo or p-hydroxo-bridged chromium(III) passivation layer is deposited on the surface of the protectivecoating. As a result, a respective passivation layer is obtained on theprotective coating.

As a matter of fact, also alternative protective coatings such aszinc-comprising coatings with iron are typically post-treated byconversion coatings (i.e. with a passivation composition) to furtherincrease corrosion resistance thereof.

However, conversion coatings typically applied to zinc coatings orzinc-comprising coatings additionally comprising nickel are notautomatically applicable to such alternative protective coatings due tothe different chemical composition of the protective coating and its owncorrosive characteristics.

In addition, conversion coatings for alternative protective coatings arealso known.

US 2006/237098 A1 refers to compositions and to a process for using saidcompositions for preparing protective coatings on various metalsubstrates. US'098 discloses a post-treatment for tin-zinc.

CN 108914106 A relates to the field of metal surface treatment liquids,in particular to a galvanized sheet surface passivation self-fillingtreatment liquid which is non-toxic and can realize self-fillinglong-term protection.

EP 2 189 551 A1 relates to a trivalent-chromium chemical conversioncoating from which substantially no hexavalent chromium is released. Itdiscloses zinc alloy coatings including zinc-iron and tin-zinc.

JP 2007 239002 A relates to a trivalent chromate liquid for treating ofa zinc-galvanized substrate, the liquid comprising a corrosion inhibitorin an amount of 0.001% to 10%.

EP 3 045 564 A1 relates a treatment liquid for a black trivalentchromium conversion coating.

Irrespective of the used protective coating, it is typically used toprotect iron and/or steel substrates, which otherwise would undergodramatic corrosion.

However, sometimes the (alternative) protective coating is slightlydamaged such that the metal substrate is at least partly exposed and notany longer completely covered by it. As a result, also the metalsubstrate dissolves in these areas while in contact with a passivationcomposition and the iron ion concentration increases over time. Itturned out that a comparatively high iron ion concentration in apassivation composition often leads to a negative coloring of thepassivated substrate or can even impair the corrosion resistance.Moreover, passivation compositions with a comparatively high iron ionconcentration must be replaced more often, which leads to higher costs.Therefore, there is a constant demand to improve existing passivationcompositions, in particular if applicable to alternative protectivecoatings, to increase the lifetime of such alternative passivationcompositions without compromising corrosion resistance.

OBJECTIVE OF THE PRESENT INVENTION

It was therefore the objective of the present invention to provide amethod for depositing a chromium-comprising passivation layer on suchalternative protective coatings, in particular comprising zinc andadditionally Fe, Sn, Mn, or mixtures thereof, which show on the one handan excellent corrosion protection and on the other hand an increasedlifetime for the passivation composition and therefore a moresustainable passivation method, even in the presence of contaminatingmetal ions such as iron ions.

SUMMARY OF THE INVENTION

The objectives mentioned above are solved by a method for depositing achromium-comprising passivation layer on a zinc-comprising coating, themethod comprising the following steps:

-   -   (a) providing a substrate comprising the zinc-comprising        coating,    -   (b) providing a passivation composition for depositing the        chromium-comprising passivation layer, the composition        comprising        -   (i) trivalent chromium ions;        -   (ii) at least one complexing agent for the trivalent            chromium ions, being different from the at least one            corrosion-inhibiting agent; and        -   (iii) 0.001 mg/L to 200 mg/L, based on the total volume of            the passivation composition, of at least one            corrosion-inhibiting agent selected from the group            consisting of unsubstituted azole compounds, substituted            azole compounds, unsubstituted aliphatic organic acids with            at least one mercapto-group, substituted aliphatic organic            acids with at least one mercapto-group, salts, and mixtures            thereof; and    -   (c) contacting said substrate with said passivation composition        such that the chromium-comprising passivation layer is deposited        on the zinc-comprising coating,    -   wherein the zinc-comprising coating additionally comprises Fe,        Sn, Mn, or mixtures thereof.

By utilizing the at least one corrosion-inhibiting agent as definedthroughout the present text in the passivation composition an excellentcorrosion protection of said zinc-comprising coating is obtained.

Moreover, by utilizing said at least one corrosion-inhibiting agent therelease of iron ions from the substrate into the passivation compositionis significantly suppressed or even fully prevented. As a result, thepassivation composition utilized in the method of the present invention,has a significantly longer lifetime compared to a passivationcomposition not comprising said at least one corrosion-inhibiting agentbut otherwise being identical.

The at least one complexing agent for the trivalent chromium ions isdifferent from the at least one corrosion-inhibiting agent. Thus (ii)and (iii) are not the same compounds but rather different compounds,which are distinct from each other.

Preferred is a method of the present invention, wherein thezinc-comprising coating is a galvanized layer or galvanic layer,respectively. This means that preferably the zinc-comprising coating iselectrolytically deposited onto the substrate from a respective coatingcomposition.

Preferred is a method of the present invention, wherein thezinc-comprising coating is preferably a zinc-alloy coating and thus thesubstrate is preferably a zinc-alloy coated substrate. In the context ofthe present invention, the zinc-comprising coating is not a purelyzinc-comprising coating or an only zinc-comprising coating. It alwayscomprises at least one of said additional metals. Furthermore, saidadditional metals are willfully added/included in the zinc-comprisingcoating, i.e. intentionally incorporated.

DETAILED DESCRIPTION OF THE INVENTION

In the context of the present invention, the term “at least one” or “oneor more” denotes (and is exchangeable with) “one, two, three or morethan three”. Furthermore, “trivalent chromium” refers to chromium withthe oxidation number +3. The term “trivalent chromium ions” refers toCr³⁺-ions in a free or complexed form.

In the context of the present invention, the term “chromium-comprisingpassivation layer” describes a layer, which comprises preferablytrivalent chromium compounds. The chromium-comprising passivation layerpreferably comprises trivalent chromium hydroxide. In some cases it ispreferred that the passivation layer comprises additional metals,preferably cobalt (i.e. most preferably cobalt compounds). The trivalentchromium compounds are preferably insoluble in water.

The Substrate Comprising the Zinc-Comprising Coating

In step (a) of the method of the present invention, a substratecomprising the zinc-comprising coating is provided, preferably azinc-comprising coating as defined throughout the present text, morepreferably defined as being preferred.

Preferred is a method of the present invention, wherein in step (a) thesubstrate comprises a metal, preferably is a metal substrate. Mostpreferably, the substrate is not a plastic substrate, preferably is notan organic substrate.

More preferred is a method of the present invention, wherein thesubstrate comprises iron (preferably is an iron substrate), preferablyas a base-material on which the zinc-comprising coating is deposited.Therefore, preferably iron ions are released from the substrate and basematerial, respectively, which in particular occurs if thezinc-comprising coating is damaged.

Preferred is a method of the present invention, wherein in step (a) thesubstrate comprises at least a fastener, preferably screws, nails, nuts,clamps and/or springs. As mentioned above, they preferably comprise orconsist of a metal.

In the method of the present invention, the zinc-comprising coatingadditionally comprises Fe, Sn, Mn, or mixtures thereof. This means, thatat least one of Fe, Sn, Mn is present together with (i.e. in additionto) zinc. In some cases it is preferred that even two (or more) of themare present together with zinc. Thus, Fe, Sn, and Mn are alloyingelements forming the galvanized layer, respectively the galvanic layer.

Preferred is a method of the present invention, wherein in thezinc-comprising coating zinc together with Fe, Sn, and Mn represent 95wt.-% or more of all metals (preferably of all elements) in thezinc-comprising coating, based on the total weight of thezinc-comprising coating, preferably 96 wt.-% or more, more preferably 97wt.-% or more, even more preferably 98 wt.-% or more, yet even morepreferably 99 wt.-% or more, most preferably 99.5 wt.-% or more, yeteven most preferably 99.9 wt.-% or more.

Thus, besides zinc, Fe, Sn, and Mn no significant further metals arepreferably involved in forming the zinc-comprising coating.

Preferred is a method of the present invention, wherein in thezinc-comprising coating zinc is present in a total amount ranging from 1wt.-% to 99.5 wt.-%, based on the total weight of the zinc-comprisingcoating, preferably ranging from 2 wt.-% to 99 wt.-%, more preferablyfrom 3 wt.-% to 95 wt.-%, even more preferably from 4 wt.-% to 93 wt.-%,yet even more preferably from 5 wt.-% to 91 wt.-%, most preferably from9 wt.-% to 85 wt.-%.

Preferred is a method of the present invention, wherein thezinc-comprising coating comprises Fe and Fe is preferably present in atotal amount ranging from 0.1 wt.-% to 35 wt.-%, based on the totalweight of the zinc-comprising coating, preferably from 0.3 wt.-% to 30wt.-%, more preferably from 0.5 wt.-% to 28 wt.-%, even more preferablyfrom 0.9 wt.-% to 26 wt.-%, yet even more preferably from 1.3 wt.-% to25 wt.-%, most preferably from 2 wt.-% to 24 wt.-%.

More preferred is a method of the present invention, wherein thezinc-comprising coating comprises Fe and Fe is preferably present in atotal amount ranging from 4 wt.-% to 35 wt.-%, based on the total weightof the zinc-comprising coating, preferably from 5 wt.-% to 30 wt.-%,more preferably from 6 wt.-% to 28 wt.-%, even more preferably from 7wt.-% to 26 wt.-%, yet even more preferably from 8 wt.-% to 25 wt.-%,most preferably from 10 wt.-% to 24 wt.-%.

Very preferred is a method of the present invention, wherein thezinc-comprising coating comprises Fe and Fe is preferably present in atotal amount ranging from 8 wt.-% to 23 wt.-%, based on the total weightof the zinc-comprising coating, preferably from 9 wt.-% to 22 wt.-%,more preferably from 10 wt.-% to 21 wt.-%, even more preferably from 11wt.-% to 20 wt.-%, yet even more preferably from 12 wt.-% to 19 wt.-%,most preferably from 13 wt.-% to 18 wt.-%. This most preferably appliesif the zinc-comprising coating is substantially free of, preferably doesnot comprise, Sn and/or (preferably and) Mn.

In other cases, preferred is a method of the present invention, whereinthe zinc-comprising coating comprises Fe and Fe is preferably present ina total amount ranging from 0.1 wt.-% to 10 wt.-%, based on the totalweight of the zinc-comprising coating, preferably from 0.2 wt.-% to 9wt.-%, more preferably from 0.3 wt.-% to 8 wt.-%, even more preferablyfrom 0.5 wt.-% to 7 wt.-%, yet even more preferably from 0.7 wt.-% to 6wt.-%, most preferably from 0.9 wt.-% to 5 wt.-%.

Preferred is a method of the present invention, wherein thezinc-comprising coating comprises Sn and Sn is preferably present in atotal amount ranging from 40 wt.-% to 95 wt.-%, based on the totalweight of the zinc-comprising coating, preferably from 50 wt.-% to 92wt.-%, more preferably from 57 wt.-% to 90 wt.-%, even more preferablyfrom 58 wt.-% to 88 wt.-%, yet even more preferably from 60 wt.-% to 86wt.-%, most preferably from 62 wt.-% to 85 wt.-%. This most preferablyapplies if the zinc-comprising coating is substantially free of,preferably does not comprise, Fe and/or (preferably and) Mn.

Preferred is a method of the present invention, wherein thezinc-comprising coating comprises Mn and Mn is preferably present in atotal amount ranging from 1 wt.-% to 60 wt.-%, based on the total weightof the zinc-comprising coating, preferably from 2 wt.-% to 50 wt.-%,more preferably from 5 wt.-% to 48 wt.-%, even more preferably from 10wt.-% to 47 wt.-%, yet even more preferably from 15 wt.-% to 45 wt.-%,most preferably from 20 wt.-% to 41 wt.-%. This most preferably appliesif the zinc-comprising coating is substantially free of, preferably doesnot comprise, Fe and/or (preferably and) Sn.

Preferred is a method of the present invention, wherein thezinc-comprising coating is substantially free of, preferably does notcomprise, nickel. This in particular denotes that the zinc-comprisingcoating does not comprise intentionally added nickel. In contrast,unavoidable nickel, for example as an impurity and/or contamination isacceptable, preferably as long as it does not show a significant effecton the zinc-comprising coating.

Preferred is a method of the present invention, wherein thezinc-comprising coating comprises Fe, Sn, or mixtures thereof,preferably Fe. This means that in such preferred cases, thezinc-comprising coating does not comprise manganese (but Fe and/or Sn),preferably does not comprise manganese and tin (but preferablyadditionally only Fe), respectively. Thus, the method of the presentinvention is most preferably for a zinc-iron coating (i.e. azinc-comprising coating additionally comprising Fe). In other cases,however, the method of the present invention is very preferably for azinc-tin coating (often also equally referred to as tin-zinc coating;i.e. a zinc-comprising coating additionally comprising tin).

The Passivation Composition

In step (b) of the method of the present invention, a passivationcomposition is provided as defined throughout the present text,preferably as defined as being preferred.

Preferred is a method of the present invention, wherein the passivationcomposition is an aqueous passivation composition, wherein preferablythe concentration of water is more than 50 vol.-%, based on the totalvolume of the passivation composition, more preferably 65 vol.-% ormore, even more preferably 80 vol.-% or more, most preferably 90 vol.-%or more.

In the method of the present invention, the passivation compositioncomprises (i) trivalent chromium ions.

Preferred is a method of the present invention, wherein the passivationcomposition comprises trivalent chromium ions in a total concentrationfrom 0.1 g/L to 25 g/L, based on the total volume of the passivationcomposition, preferably from 0.2 g/L to 20 g/L, more preferably from 0.3g/L to 15 g/L, even more preferably from 0.4 g/L to 10 g/L, mostpreferably from 0.5 g/L to 9 g/L. If the total concentration issignificantly below 0.1 g/L typically an insufficient passivation isobtained. On the other hand, if the total concentration is significantlyexceeding 25 g/L, the entire method is not sufficiently ecological.

In some cases, very preferred is a method of the present invention,wherein the passivation composition comprises trivalent chromium ions ina total concentration from 0.5 g/L to 3 g/L, based on the total volumeof the passivation composition, preferably from 1 g/L to 2.5 g/L. Withsuch concentrations, very excellent results were obtained.

In the method of the present invention, the passivation compositioncomprises (ii) at least one complexing agent for the trivalent chromiumions.

Preferred is a method of the present invention, wherein the at least onecomplexing agent for the trivalent chromium ions is selected from thegroup consisting of organic complexing agents and inorganic complexingagents. The proviso still applies that the organic complexing agent isdifferent from the at least one corrosion-inhibiting agent as definedthroughout the present text.

Preferably, said at least one complexing agent is not only for thetrivalent chromium ions but is additionally also a complexing agent foriron ions, most preferably for released iron ions. If they are alsocomplexed, an undesired sludge formation is strongly reduced or evenprevented.

Preferred is a method of the present invention, wherein in thepassivation composition the at least one complexing agent for thetrivalent chromium ions is selected from the group consisting ofmonocarboxylic acids, dicarboxylic acids, salts thereof (of bothmonocarboxylic acids and dicarboxylic acids), halogen ions, and mixturesthereof.

Most preferably, the at least one complexing agent comprises at leastone dicarboxylic acid and/or salts thereof.

Preferred is a method of the present invention, wherein in thepassivation composition the at least one complexing agent for thetrivalent chromium ions is selected from the group consisting ofunsubstituted monocarboxylic acids, hydroxyl-substituted monocarboxylicacids, amino-substituted monocarboxylic acids, unsubstituteddicarboxylic acids, hydroxyl-substituted dicarboxylic acids,amino-substituted dicarboxylic acids, salts thereof (of allaforementioned acids), halogen ions, and mixtures thereof.

Preferred is a method of the present invention, wherein the at least onecomplexing agent for the trivalent chromium ions is selected from thegroup consisting of oxalate/oxalic acid, acetate/acetic acid,tartrate/tartaric acid, malate/malic acid, succinate/succinic acid,gluconate/gluconic acid, glutamate/glutamic acid, glycolate/glycolicacid, diglycolate/diglycolic acid, ascorbate/ascorbic acid, andbutyrate/butyric acid.

Preferred is a method of the present invention, wherein the halogen ionscomprise fluoride ions, preferably are fluoride ions, most preferablyare only fluoride ions out of halogen ions.

Preferred is a method of the present invention, wherein the at least onecomplexing agent for the trivalent chromium ions does not comprise amercapto-group.

Preferred is a method of the present invention, wherein the at least onecomplexing agent for the trivalent chromium ions does not comprise atriazole, preferably not an azole, more preferably not an aromaticorganic compound.

In some cases preferred is a method of the present invention, whereinthe at least one complexing agent for the trivalent chromium ions has atotal concentration in a range from 0.01 mol/L to 2 mol/L, based on onemol/L trivalent chromium ions in the passivation composition, preferablyfrom 0.03 mol/L to 1 mol/L, more preferably from 0.05 mol/L to 0.8mol/L, even more preferably from 0.1 mol/L to 0.5 mol/L.

Also preferred is a method of the present invention, wherein in thepassivation composition the at least one complexing agent for thetrivalent chromium ions has a total concentration in a range from 0.05wt.-% to 15 wt.-%, based on the total weight of the passivationcomposition, preferably from 0.1 wt.-% to 10 wt.-%, more preferably from0.2 wt.-% to 9 wt.-%, even more preferably from 0.5 wt.-% to 8 wt.-%,most preferably from 0.8 wt.-% to 7 wt.-%.

Typically, in the above defined (preferred) concentration ranges thetrivalent chromium ions are efficiently stabilized in the passivationcomposition by the complexing agents (preferably complexing agents asdefined as being preferred).

In the method of the present invention, the passivation compositioncomprises (iii) 0.001 mg/L to 200 mg/L, based on the total volume of thepassivation composition, of at least one corrosion-inhibiting agent asdefined throughout the present text, preferably as defined as beingpreferred. This concentration range refers to a total concentration andis to be understood as (and preferably exchangeable with) “(iii) 0.001mg/L to 200 mg/L in total, based on [. . . ]”. This preferably alsoapplies to all preferred concentration ranges defined throughout thepresent text as being preferred.

If (iii) is present significantly above 200 mg/L in most cases aninsufficient corrosion resistance is obtained, which means that in forexample a NSS test undesired results are obtained (although an excellentinhibiting effect might be achieved; see examples below). In order toachieve an inhibiting effect at all, a comparatively low concentrationis already sufficient (see examples below), at least 0.001 mg/L,preferably at least 0.01 mg/L, more preferably at least 0.1 mg/L, whichare preferably individually combined with said 200 mg/L or other upperlimits.

Preferred is a method of the present invention (preferably as describedas being preferred throughout the present text), wherein in thepassivation composition (iii) has a total concentration ranging from0.005 mg/L to 180 mg/L, based on the total volume of the passivationcomposition, preferably ranging from 0.01 mg/L to 160 mg/L, morepreferably ranging from 0.1 mg/L to 150 mg/L, even more preferablyranging from 1 mg/L to 135 mg/L, yet even more preferably ranging from 2mg/L to 120 mg/L, most preferably ranging from 3 mg/L to 110 mg/L.

Preferred is a method of the present invention (preferably as describedas being preferred throughout the present text), wherein in thepassivation composition (iii) has a total concentration ranging from0.01 mg/L to 150 mg/L, based on the total volume of the passivationcomposition, preferably ranging from 0.05 mg/L to 120 mg/L, morepreferably ranging from 0.1 mg/L to 100 mg/L, even more preferablyranging from 0.5 mg/L to 80 mg/L, yet even more preferably ranging from1 mg/L to 50 mg/L, most preferably ranging from 2 mg/L to 25 mg/L. Insome cases it is very preferred that these preferred concentrationranges apply in particular to corrosion-inhibiting agents selected fromthe group consisting of unsubstituted aliphatic organic acids with atleast one mercapto-group, substituted aliphatic organic acids with atleast one mercapto-group, salts, and mixtures thereof; in particular asfurther defined as being preferred throughout the present text.

Preferred is a method of the present invention (preferably as describedas being preferred throughout the present text), wherein in thepassivation composition (iii) has a total concentration ranging from0.001 mg/L to 9.9999 mg/L, based on the total volume of the passivationcomposition, preferably ranging from 0.01 mg/L to 9.9 mg/L, morepreferably ranging from 0.1 mg/L to 9.8 mg/L, even more preferablyranging from 0.5 mg/L to 9.7 mg/L, yet even more preferably ranging from0.8 mg/L to 9.6 mg/L, most preferably ranging from 1 mg/L to 9.5 mg/L,even most preferably ranging from 2 mg/L to 9.4 mg/L. In some cases itis very preferred that these preferred concentration ranges apply inparticular to corrosion-inhibiting agents selected from the groupconsisting of unsubstituted azole compounds, substituted azolecompounds, salts, and mixtures thereof; in particular as further definedas being preferred throughout the present text.

Most preferred is a method of the present invention, wherein in thepassivation composition (iii) has a total concentration ranging from0.001 mg/L to 9 mg/L, based on the total volume of the passivationcomposition, preferably ranging from 0.01 mg/L to 8.8 mg/L, morepreferably ranging from 0.1 mg/L to 8.5 mg/L, even more preferablyranging from 0.5 mg/L to 8.3 mg/L, yet even more preferably ranging from0.8 mg/L to 8 mg/L, most preferably ranging from 1 mg/L to 7.5 mg/L, andeven most preferably ranging from 2 mg/L to 7 mg/L. Own experiments haveshown that typically such low concentrations result in excellent resultsand are already sufficient (see examples below) although higherconcentrations can be used too. It was surprising that a verysignificant effect was already obtained with a comparatively low totalconcentration.

In the passivation composition the at least one corrosion-inhibitingagent is selected from the group consisting of unsubstituted azolecompounds, substituted azole compounds (which are preferred azolecompounds), unsubstituted aliphatic organic acids with at least onemercapto-group (which are preferred aliphatic organic acids),substituted aliphatic organic acids with at least one mercapto-group,salts, and mixtures thereof.

Preferred is a method of the present invention, wherein in thepassivation composition the substituted azole compounds, salts, andmixtures thereof independently comprise one or more than one substituentselected from the group consisting of amino, nitro, carboxy, hydroxy,sulfonate, and thiol, wherein preferably the substituent is a thiolgroup.

Preferred is a method of the present invention, wherein in thepassivation composition the unsubstituted, and substituted azolecompounds, salts, and mixtures thereof are independently selected fromthe group consisting of monoazoles, diazoles, triazoles, and tetrazoles,preferably diazoles and triazoles, most preferably triazoles.

Preferred is a method of the present invention, wherein in thepassivation composition the unsubstituted and substituted (preferablythe substituted) azole compounds, salts, and mixtures thereof areindependently selected from the group consisting of 1,2,4-triazoles.This most preferably means 1,2,4-H-triazoles.

Preferred is a method of the present invention, wherein in thepassivation composition the substituted azole compounds, salts, andmixtures thereof independently comprise at least a mercaptotriazole,preferably at least 3-mercapto-1,2,4-triazole (most preferably denoting3-mercapto-1,2,4-H-triazole).

In the method of the present invention, the at least onecorrosion-inhibiting agent can also be a substituted aliphatic organicacid with at least one mercapto-group, salts, and mixtures thereof. Inthis case, the organic acid is substituted by means of one or more thanone substituent. Preferably, the acid is a carboxylic acid, a sulfonicacid, salts, or mixtures thereof. The respective acid moiety, i.e.preferably the carboxy group(s) and sulfonic group(s) is (are) not thesubstituent because it characterizes the compound as acid. In contrast,preferred is a method of the present invention, wherein in thepassivation composition the aliphatic organic acid with at least onemercapto-group, salts, and mixtures thereof independently comprise oneor more than one substituent (i.e. in addition to the acid moieties)selected from the group consisting of amino, nitro, and hydroxy.

Preferred is a method of the present invention, wherein in thepassivation composition the unsubstituted and substituted aliphaticorganic acids with at least one mercapto-group and salts thereof,respectively, comprise a carboxylic acid and/or salts thereof.

More preferred is a method of the present invention, wherein in thepassivation composition the unsubstituted and substituted aliphaticorganic acids with at least one mercapto-group and salts thereof,respectively, comprise a monocarboxylic acid and/or salts thereof.

Preferred is a method of the present invention, wherein in thepassivation composition the unsubstituted and substituted aliphaticorganic acids with at least one mercapto-group and salts thereof,respectively, comprise 1 to 12 carbon atoms, preferably 2 to 10 carbonatoms, more preferably 3 to 8 carbon atoms, most preferably 3 to 6carbon atoms.

In some cases, very preferred is a method of the present invention,wherein (iii) comprises at least one unsubstituted aliphatic organicacid with at least one mercapto-group and/or salts thereof.

Preferred is a method of the present invention, wherein in thepassivation composition the unsubstituted aliphatic organic acids withat least one mercapto-group, salts, and mixtures thereof comprise atleast 3-mercaptopropionic acid and/or salts thereof, most preferably is3-mercaptopropionic acid.

Generally preferred is a method of the present invention, wherein (iii)comprises at least one of said azole compounds (including compoundsdefined as being preferred) or at least one of said aliphatic organicacids (including compounds defined as being preferred). However, in somecases preferred is a method of the present invention, wherein there is amixture of at least one of said azole compounds with at least one ofsaid aliphatic organic acids is present in the passivation composition.

In some cases, preferred is a method of the present invention, whereinthe passivation composition further comprises (iv) divalent cobalt ions,preferably in a total concentration from 0.01 g/L to 5 g/L, based on thetotal volume of the passivation composition, preferably from 0.1 g/L to4 g/L, more preferably from 0.3 g/L to 3.5 g/L, even more preferablyfrom 0.5 g/L to 3 g/L, most preferably from 0.7 g/L to 2.5 g/L.

In many cases, cobalt ions positively affect an optional heat-treating.In the context of the present invention, heat-treating as apost-treatment is an optional step (for heat-treating see text below).

However, in some cases a method of the present invention is preferred,wherein the passivation composition is essentially free of, preferablydoes not comprise, divalent cobalt ions; preferably is essentially freeof, preferably does not comprise, any cobalt ions; most preferably isessentially free of, preferably does not comprise, cobalt at all. Byalternatively excluding cobalt or cobalt ions, respectively, from thepassivation composition, a cost-reduction is typically achieved, sincethe use of expensive cobalt compounds is avoided, and the wastewatertreatment is simplified, thereby improving the sustainability of thepassivation composition. Furthermore, in many cases cobalt and cobaltions, respectively, do not result in any benefit and can thereforeomitted.

Preferred is a method of the present invention, wherein the passivationcomposition is essentially free of, preferably does not comprise,intentionally added hexavalent chromium compounds. In other words, ifhexavalent chromium compounds are present, they are a result oftypically undesired reactions of the trivalent chromium ions. Thus, amethod of the present invention is preferred, with the proviso, ifhexavalent chromium ions are present, they originate from said trivalentchromium ions.

Preferred is a method of the present invention, wherein the passivationcomposition has a pH ranging from 0.5 to 6.5, preferably from 0.7 to 6,more preferably from 0.9 to 5, even more preferably from 1.1 to 4, yeteven more preferably from 1.4 to 3, most preferably from 1.6 to 2.7, yeteven most preferably from 1.8 to 2.3. If the pH is significantlyexceeding 6.5, in some cases undesired precipitation is observed. If thepH is significantly below 0.5, in some cases an undesired and too strongdissolution of the substrate is observed; in particular if ironsubstrates are used. The preferred pH ranges as defined above are inparticular beneficial for effectively depositing the chromium-comprisingpassivation layer and to maintain a comparatively long lifetime of thepassivation composition.

Preferred is a method of the present invention, wherein the passivationcomposition further comprises

-   -   (v) iron ions in a total concentration ranging from 0 mg/L to        2000 mg/L, based on the total volume of the passivation        composition, preferably ranging from 0 mg/L to 500 mg/L, more        preferably ranging from 0 mg/L to 300 mg/L, most preferably        ranging from 0 mg/L to 250 mg/L, even most preferably ranging        from 0 mg/L to 200 mg/L.

Preferred is a method of the present invention, wherein the iron ions,if present, are from the substrate.

In some cases, iron ions are (almost) permanently present, preferably ata very low concentration, most preferably not even reaching the upperconcentration limits as defined above. In view of the present invention,in many cases the upper concentration limits as defined above are notcritical, in particular if the total concentration is not exceeding 500mg/L. Typical very low concentrations of the iron ions are preferably0.001 mg/L or more, based on the total volume of the passivationcomposition, more preferably 0.01 mg/L, even more preferably 0.1 mg/L,and most preferably 1 mg/L. Preferably, such low concentrations arefreely combined with the upper concentration limits defined above.Without wishing to be bound by theory, it appears that the at least onecorrosion-inhibiting agent primarily minimizes the release of iron ionsbut it cannot be ruled out completely that it also positively interactswith released iron ions such that a respective passivation compositionhas a prolonged lifetime.

Contacting the Substrate With the Passivation Composition

In step (c) of the method of the present invention, the substrate iscontacted with the passivation composition such that thechromium-comprising passivation layer is deposited.

Most preferred is a method of the present invention, wherein step (c) isperformed without applying an electrical current, i.e. in the absence ofan electrical current.

Preferred is a method of the present invention, wherein step (c) isperformed at a temperature in a range from 10° C. to 90° C., preferablyfrom 13° C. to 70° C., more preferably from 15° C. to 60° C., even morepreferably from 20° C. to 50° C., yet even more preferably from 23° C.to 45° C., most preferably from 26° C. to 40° C. Such temperatures allowan effective and/or sustainable operation of the method of the presentinvention.

If the temperature is significantly exceeding 90° C., in some cases anundesired evaporation of water is observed along with an undesiredconsumption of energy. If the temperature is significantly below 10° C.,in many cases an insufficient depositing of the chromium-comprisingpassivation layer is obtained, thereby compromising the quality of thecorrosion protection.

Preferred is a method of the present invention, wherein step (c) isperformed for a time period from 5 seconds to 600 seconds, preferablyfrom 10 seconds to 400 seconds, more preferably from 15 seconds to 300seconds, even more preferably from 20 seconds to 200 seconds, mostpreferably from 25 seconds to 150 seconds. If the time period issignificantly below 5 seconds, in many cases an insufficient depositingof the chromium-comprising passivation layer is obtained, therebycompromising the quality of the corrosion protection.

By performing step (c) in the preferred temperature ranges and thepreferred time periods, particularly advantageous deposition kineticsare obtained. In many cases, exceeding the specified longest period oftime and highest specified temperature, no further benefits are usuallyobtained.

Preferred is a method of the present invention, wherein after step (c)(preferably after a step (c)) the passivation composition comprises ironions in a concentration of 200 mg/L or less, based on the total volumeof the passivation composition, preferably 100 mg/L or less, mostpreferably 200 mg/L or less after each step (c), even most preferably100 mg/L or less after each step (c).

More preferred is a method of the present invention, wherein after step(c) (preferably after a step (c)) the passivation composition comprisesiron ions in a concentration of 500 mg/L or less, based on the totalvolume of the passivation composition, preferably 400 mg/L or less, morepreferably 300 mg/L or less, most preferably 250 mg/L or less, even mostpreferably 200 mg/L or less, each with the proviso that the passivationcomposition comprises zinc ions with a concentration of 15 g/L or less.

Even more preferred is a method of the present invention, wherein afterstep (c) (preferably after a step (c)) the passivation compositioncomprises iron ions in a concentration of 500 mg/L or less, based on thetotal volume of the passivation composition, preferably 400 mg/L orless, more preferably 300 mg/L or less, most preferably 250 mg/L orless, even most preferably 200 mg/L or less, each with the proviso thatthe passivation composition comprises zinc ions with a concentration of10 g/L or less.

Preferred is a method of the present invention, wherein the methodcomprises after step (c), additionally step

(d) heat-treating the substrate with the chromium-comprising passivationlayer.

Typically, the heat-treating minimizes hydrogen embrittlement.

Preferred is a method of the present invention, wherein in step (d) theheat-treating is performed at a temperature in a range from 150° C. to230° C., preferably from 180° C. to 210° C.

Preferred is a method of the present invention, wherein in step (d) theheat-treating is performed for a time period from 1 hour to 10 hours,preferably from 2 hours to 8 hours, most preferably from 2.5 hours to 5hours.

Most preferred is a method of the present invention, wherein after step(c) or (d), preferably (c), the substrate with the chromium-comprisingpassivation layer has a white rust formation of 1% or below if testedaccording to DIN 9227. A white rust formation of 1% or below accordingto DIN 9227 serves as a particularly good criteria for proving theexcellent corrosion protection obtained with the method of the presentinvention.

In some cases, preferred is a method of the present invention, whereinthe method comprises after step (c) or (d), additionally step

(e) sealing the substrate with the chromium-comprising passivation layerwith a sealing layer such that a sealed substrate is obtained.

Preferred is a method of the present invention, wherein the sealinglayer comprises a compound (or a reaction product thereof) selected fromthe group consisting of inorganic silicates (preferably as particles),silanes, organic polymers, and mixtures thereof.

Regarding aforementioned inorganic silicates (preferably as particles),alternatively or in addition, such particles are preferably comprised inthe passivation composition utilized in the method of the presentinvention in order to further increase corrosion resistance.

The Chromium-Comprising Passivation Layer

In step (c) of the method of the present invention, thechromium-comprising passivation layer is deposited.

Preferred is a method of the present invention, wherein thechromium-comprising passivation layer has a layer thickness in a rangefrom 1 nm to 1200 nm, preferably from 10 nm to 1000 nm, more preferablyfrom 15 nm to 800 nm, most preferably from 20 nm to 500 nm.

Preferred is a method of the present invention, wherein thechromium-comprising passivation layer is a bluish, preferably a blue,chromium-comprising passivation layer.

Even more preferred is a method of the present invention, wherein thechromium-comprising passivation layer is a bluish, preferably a blue,chromium-comprising passivation layer and has a layer thickness in arange from 30 nm to 150 nm, preferably from 40 nm to 140 nm, morepreferably from 45 nm to 130 nm, most preferably from 50 nm to 120 nm,and even most preferably from 55 nm and 90 nm.

In a few cases a method of the present invention is preferred, whereinthe chromium-comprising passivation layer is iridescent and has a layerthickness in a range from 155 nm to 1200 nm, preferably from 170 nm to1000 nm, more preferably from 190 nm to 800 nm, most preferably from 200nm to 600 nm.

In a few cases a method of the present invention is preferred, whereinthe chromium-comprising passivation layer is transparent or yellow andhas a layer thickness in a range from 1 nm to 25 nm, preferably from 3nm to 22 nm, more preferably from 5 nm to 20 nm, most preferably from 8nm to 18 nm.

The present invention is described in more detail by the followingnon-limiting examples.

EXAMPLES 1. First Set of Experiments (Inhibitor Test)

In a first set of experiments it was investigated whether3-Mercaptotriazole (3-MTA) and 3-Mercaptopropionic acid (3-MPA) aresuitable to prevent iron dissolution from iron substrates. For that,aqueous test passivation compositions were prepared, comprising about 2g/L trivalent chromium ions, optional cobalt ions, a dicarboxylic acidas complexing agent, and either (3-MTA) or (3-MPA), respectively, ascorrosion-inhibiting agent. In a control experiment, nocorrosion-inhibiting agent was used. The pH in each case was about 2.2to 2.5.

In each experiment an iron substrate (a 3.5 cm×5.0 cm iron plate) wastested in a beaker for 2 hours to allow the test passivationcompositions to dissolve the iron substrate.

The test procedure was as follows: After setting up each testpassivation composition the pH was adjusted as mentioned before. Duringeach test, the pH was monitored. A final pH was measured after the 2hours. It was observed that the pH remained stable if thecorrosion-inhibiting agent was successfully minimizing the irondissolution. However, if the total amount of the corrosion-inhibitingagent was too low or consumed over time, the pH increased, typicallyexceeding a pH of 3.0, even up to 3.5. A stable pH is considered as“passing” the test, wherein an increased pH up to 3.0 or even more isconsidered as “failed”. The results are summarized in Table 1 below.

TABLE 1 Exp. 3-MTA [mg/L] 3-MPA [mg/L] result A 0 0 Failed B 7 0 PassedC 100 0 Passed D 1000 0 Passed E 0 7 Passed F 0 100 Passed G 0 1000Passed

The inhibitor test clearly shows that both 3-MTA and 3-MPA areeffectively minimizing the dissolution of iron. In the absence of both,a significant pH increase was observed (see Experiment A). Thus, in theabsence of a corrosion-inhibiting agent, an undesired release of ironions occurred.

In contrast, in the presence of a corrosion-inhibiting agent the pHremained stable over said 2 hours. Thus, no significant iron dissolutionoccurred. Furthermore, no significant difference was observed between acomparatively low concentration (e.g. 7 mg/L) and a comparatively highconcentration (e.g. 100 mg/l and 1000 mg/L, respectively) of acorrosion-inhibiting agent. In each case a stable pH was observed,indicating a sufficiently minimized iron dissolution.

Furthermore, it appears that the presence or absence of cobalt has nosignificant influence on the iron dissolution. Also, there seems to beno significant advantage or disadvantage of having cobalt in combinationwith a corrosion-inhibiting agent.

Similar results and conclusions were obtained with5-Mercapto-1-methyltetrazole and benzotriazole (data not shown) withslightly less good results for benzotriazole.

2. Second Set of Experiments (Zn-Comprising Coating AdditionallyComprising Fe; Corrosion Resistance Test)

In this second test (and the following third test below) the effect ofthe corrosion-inhibiting agent in the passivation composition on thecorrosion resistance after passivation was investigated.

For this set of experiments, aqueous test passivation compositions withthe numbering as introduced in Table 2 were prepared, being identical tothe test passivation compositions as used for the first set ofexperiments. No iron ions were actively/intentionally added nor expecteddue to dissolution from the substrates because of the short utilizationof the passivation composition.

The method of the present invention was carried out as follows: Assubstrates a plurality of ZnFe coated iron screws (M8x60; about 12 wt.-%Fe in the ZnFe) were pre-treated and subsequently passivated for 30seconds in the respective aqueous test passivation compositions (volumeeach: 2 L) at room temperature (appr. 20° C.). Afterwards the passivatedscrews were optically inspected and subjected to a NSS test. The opticalinspection was in all cases passed.

Further details regarding the passivation compositions and the resultsobtained after passivating the screws are summarized in Table 2 below.

TABLE 2 Exp. 3-MTA [mg/L] 3-MPA [mg/L] NSST [24/96/168] 1 7 0 +/+/− 2100 0 0/−/− 3 0 7 +/+/+ 4 0 100 0/−/− C1 0 0 +/+/+ C2 300 0 −/−/− C31000 0 −/−/− C4 0 300 −/−/− C5 0 1000 −/−/−

In Table 2, “NSST [24/96/168]” denotes neutral salt spray test accordingto DIN 9227 with a duration of 24, 96, and 168 hours, wherein “+”denotes that the test was passed with no or (in only a few cases) anacceptable degree of white rust formation; “0” denotes an undesireddegree of white rust formation on almost all tested screws but stillacceptable (basically no red rust formation); and “−” denotes aninacceptable degree of white rust formation on all screws including insome cases even a significant red rust formation.

Experiments 1 to 4 are experiments according to the invention, whereinexperiments C1 to C5 are comparative experiments.

As mentioned above, no intentionally added iron ions were present in theaqueous test passivation compositions. It is generally desired to avoidiron ions in a passivation composition because it is typically observedthat they negatively affect the corrosion resistance after passivation,e.g. in a NSS test. In addition, with increasing amounts of iron ionsthe lifetime of a respective passivation composition is significantlyshortened.

Experiments 1 to 4 and C1 to C5 clearly show how the corrosionresistance after passivation is affected in the presence of varyingconcentrations of 3-MTA and 3-MPA, respectively. It was generallyobserved that with increasing concentration of 3-MTA and 3-MPA,respectively, the corrosion resistance decreases. As shown, the totalconcentration of 3-MTA and 3-MPA, respectively, should never reach oreven exceed 300 mg/L because a dramatic detrimental effect on corrosionresistance was observed (as well as for 1000 mg/L). In contrast, acomparatively low total concentration of 7 mg/L does not at allnegatively affect corrosion resistance and can be easily tolerated suchthat the positive inhibitory effect can be achieved without compromisingthe corrosion resistance quality. If the total concentration is 100mg/L, in many cases a still acceptable corrosion resistance is obtainedinitially and after 24 hours NSS testing. However, the corrosionresistance is reduced for 96 and 168 hours NSS testing, respectively(see Table 2, Experiments 2 and 4).

Since no iron ions were intentionally added, comparative example C1showed excellent results even in the absence of 3-MTA and 3-MPA,respectively. This example represents an ideal situation, in which noiron ion contamination was present; thus, no corrosion inhibitor must beutilized. However, such ideal situation does not represent a realday-to-day situation, wherein an increased iron ion contamination istypically observed; mainly because of defective zinc-comprisingcoatings.

Although 3-MTA and 3-MPA in concentrations of even up to 1000 mg/L wellminimize the release of iron ions (see Table 1, Experiments D and G),Table 2 clearly shows that such comparatively high concentrations (i.e.300 mg/L and 1000 mg/L, respectively) negatively affect the corrosionresistance of respective passivated substrates (see Table 2, ExperimentsC2 to C5).

As a result, the total concentration of the corrosion-inhibiting agentutilized in the method of the present invention must be well balanced inorder to achieve a sufficient minimization of iron dissolution on theone hand and to avoid a reduced corrosion resistance of the passivatedsubstrate on the other hand.

3. Third Set of Experiments (Zn-Comprising Coating AdditionallyComprising Sn; Corrosion Resistance Test)

In this third test again the effect of the corrosion-inhibiting agent inthe passivation composition on the corrosion resistance afterpassivation was investigated.

For this set of experiments, aqueous test passivation compositions asdefined for the first and second set of experiments were used. Again, noiron ions were intentionally added nor expected due to dissolution fromthe substrates because of the short utilization of the passivationcomposition.

The method of the present invention was carried out as defined for thesecond set of experiments with the difference that the screws weregalvanized with a zinc-comprising coating additionally comprising Sn(about 60 wt.-% Sn in the ZnSn) instead of Fe. Further details regardingthe passivation compositions and the results obtained after passivatingthe screws are summarized in Table 3 below.

TABLE 3 Exp. 3-MTA [mg/L] 3-MPA [mg/L] NSST 5 7 0 + 6 100 0 0 7 0 7 + 80 100 0 C6  0 0 + C7  300 0 − C8  1000 0 − C9  0 300 − C10 0 1000 −

In Table 2, “NSST” denotes neutral salt spray test according to DIN 9227with a duration of 24 hours, wherein “+” denotes that the test waspassed including a still acceptable white rust formation; “0” denotes asignificantly increased degree of white rust formation compared to “+”on almost all tested screws but still acceptable (basically no red rustformation occurred); and “−” denotes an inacceptable degree of whiterust formation on all screws including in some cases even a significantred rust formation.

Experiments 5 to 8 are experiments according to the invention, whereinexperiments C6 to C10 are comparative experiments.

In a few cases, the passivation quality of ZnFe-coated substrates wasbetter/higher than for ZnSn-coated substrates. However, the sameconclusion as for ZnFe-coated substrates applies likewise to ZnSn-coatedsubstrates. Under ideal circumstances, no corrosion-inhibiting agent isneeded. However, in real day-to-day situations an iron ion increase istypically observed over time such that a corrosion-inhibiting agent isof great advantage. If such a corrosion-inhibiting agent is utilized,the total concentration thereof must be carefully balanced; too highconcentrations (i.e. again 300 mg/L and 1000 mg/L, respectively) show anegative impact on the over-all corrosion resistance after passivation.

1. A method for depositing a chromium-comprising passivation layer on azinc-comprising coating, the method comprising the following steps: (a)providing a substrate comprising the zinc-comprising coating, (b)providing a passivation composition for depositing thechromium-comprising passivation layer, the composition comprising (i)trivalent chromium ions; (ii) at least one complexing agent for thetrivalent chromium ions, being different from the at least onecorrosion-inhibiting agent; and (iii) 0.001 mg/L to 200 mg/L, based onthe total volume of the passivation composition, of at least onecorrosion-inhibiting agent selected from the group consisting ofunsubstituted azole compounds, substituted azole compounds,unsubstituted aliphatic organic acids with at least one mercapto-group,substituted aliphatic organic acids with at least one mercapto-group,salts, and mixtures thereof; and (c) contacting said substrate with saidpassivation composition such that the chromium-comprising passivationlayer is deposited on the zinc-comprising coating, wherein thezinc-comprising coating additionally comprises Fe, Sn, Mn, or mixturesthereof.
 2. The method of claim 1, wherein the zinc-comprising coatingis substantially free of nickel.
 3. The method of claim 1, wherein thezinc-comprising coating comprises Fe, Sn, or mixtures thereof.
 4. Themethod of claim 1, wherein in the passivation composition (iii) has atotal concentration ranging from 0.005 mg/L to 180 mg/L, based on thetotal volume of the passivation composition.
 5. The method of claim 1,wherein in the passivation composition (iii) has a total concentrationranging from 0.01 mg/L to 150 mg/L, based on the total volume of thepassivation composition.
 6. The method of claim 1, wherein in thepassivation composition (iii) has a total concentration ranging from0.001 mg/L to 9.9999 mg/L, based on the total volume of the passivationcomposition.
 7. The method of claim 1, wherein in the passivationcomposition the substituted azole compounds, salts, and mixtures thereofindependently comprise one or more than one substituent selected fromthe group consisting of amino, nitro, carboxy, hydroxy, sulfonate, andthiol.
 8. The method of claim 1, wherein in the passivation compositionthe unsubstituted, and substituted azole compounds, salts, and mixturesthereof are independently selected from the group consisting ofmonoazoles, diazoles, triazoles, and tetrazoles.
 9. The method of claim1, wherein in the passivation composition the unsubstituted andsubstituted azole compounds, salts, and mixtures thereof areindependently selected from the group consisting of 1,2,4-triazoles. 10.The method of claim 1, wherein in the passivation composition thesubstituted azole compounds, salts, and mixtures thereof independentlycomprise at least a mercaptotriazole.
 11. The method of claim 1, whereinin the passivation composition the unsubstituted aliphatic organic acidswith at least one mercapto-group, salts, and mixtures thereof compriseat least 3-mercaptopropionic acid and/or salts thereof.
 12. The methodof claim 1, wherein in the passivation composition the at least onecomplexing agent for the trivalent chromium ions is selected from thegroup consisting of monocarboxylic acids, dicarboxylic acids, saltsthereof, halogen ions, and mixtures thereof.
 13. The method of claim 1,wherein in the passivation composition the at least one complexing agentfor the trivalent chromium ions is selected from the group consisting ofunsubstituted monocarboxylic acids, hydroxyl-substituted monocarboxylicacids, amino-substituted monocarboxylic acids, unsubstituteddicarboxylic acids, hydroxyl-substituted dicarboxylic acids,amino-substituted dicarboxylic acids, salts thereof, halogen ions, andmixtures thereof.
 14. The method of claim 1, wherein in the passivationcomposition the at least one complexing agent for the trivalent chromiumions has a total concentration in a range from 0.05 wt.-% to 15 wt.-%,based on the total weight of the passivation composition.
 15. The methodof claim 1, wherein the passivation composition further comprises (v)iron ions in a total concentration ranging from 0 mg/L to 2000 mg/L,based on the total volume of the passivation composition.